Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Optical coherence tomography (OCT) is a widely used interferometric technique for studying biological samples including in vivo tissue, with lateral and depth resolution, using information contained within the amplitude and phase of reflected light. OCT systems generally utilise a Michelson interferometer configuration, with two main approaches being employed: time domain OCT and spectral domain OCT. As shown in FIG. 1, in a typical time domain OCT system light from a broadband optical source 2 such as a superluminescent light emitting diode (SLED) is split with a beamsplitter 4 into a reference beam 6 and a sample beam 8. Back-reflected light from a reference mirror 10 and one or more layers within a sample 12 are recombined by the beamsplitter 4, and the interference signal 14 captured with a photodetector 16. An interference signal 14 is only obtained when the round trip optical path lengths of the sample and reference arms are matched to within the coherence length of the broadband source 2, which should be small (e.g. of order a few microns) to provide good depth resolution. The optical path length difference between the sample and reference arms is scanned by axial movement 17 of the reference mirror 10, and the recorded interference signal demodulated to provide a depth-resolved profile of reflectivity within the sample 12, commonly known as an ‘A scan’.
Spectral domain OCT systems are classified as either spectrometer based or swept source based. FIG. 2 shows a typical spectrometer based system, which differs from the time domain OCT system of FIG. 1 in that instead of scanning the reference mirror 10, the interference signal 14 is dispersed with a diffraction grating 18 or other spectral demultiplexer and the various wavelengths detected simultaneously along a linear photodetector array 20. As shown in FIG. 3, swept source (SS) systems have a wavelength tuneable or steppable light source 22, such as an external cavity frequency tuned semiconductor laser, with a narrow instantaneous spectral line width 23 that is scanned in time either continuously or in discrete steps. In this case the spectrum of the interference signal 14 is recorded by a photodetector 16 as the wavelength of the light source is scanned. Basic principles of SS OCT systems are described in U.S. Pat. No. 5,956,355 entitled ‘Method and apparatus for performing optical measurements using a rapidly frequency-tuned laser’. In both types of spectral domain OCT system phase information from various reflective layers in the sample 12 is encoded in the interference spectrum, so that a depth-resolved reflectivity profile (i.e. an A scan) of the sample is extracted with a Fourier transform. Generally speaking, both types of spectral domain OCT systems have a ˜20 to 30 dB sensitivity advantage over time domain OCT systems.
OCT systems can be made polarisation sensitive by the use of a polarising beam splitter in place of the conventional power beam splitter 4, and the addition of various polarising optical elements. A key feature of polarisation sensitive OCT is that in addition to being able to discriminate between different tissue types based on the reflected light, the birefringence properties of the sample in transmission to and from the reflection points can be analysed and used as a contrast-enhancing feature. In many biological systems for example, muscle tissue and fibrous tissues often have different polarisation features compared to fatty and malignant tissues.
OCT techniques can be adapted to provide a laterally resolved ‘B scan’ by scanning the sample beam 8 relative to the sample 12 in one or two axes. Faster A scan and B scan acquisition is generally desirable and has been greatly improved over the previous 20 to 25 years by advances in several fields including faster swept source scanning rates and photodetector array readout speeds. However a fundamental limitation with scanning spot schemes, especially for in vivo applications, is presented by laser safety regulations: reducing dwell time to increase scanning speed without being able to increase the applied power will inevitably degrade the signal to noise ratio. Additionally, high speed swept wavelength laser systems are expensive and can be very difficult to use effectively. For example high speed Fourier Domain Mode Locked (FDML) lasers, which have been used in many high speed OCT demonstrations to date, require extremely precise synchronisation of filters and control of the dispersion properties and polarisation state of a fibre ring laser. There is a need therefore to enhance acquisition times to reduce motion artefacts in samples without increasing significantly the complexity of the lasers and electronics.
Consequently there has also been research into ‘parallelised’ OCT systems in which an extended sample area is probed with lateral resolution, or an array of sample spots probed simultaneously. It is relatively straightforward to parallelise time domain OCT, e.g. by utilising a CCD camera and imaging optics as described in U.S. Pat. No. 5,465,147 entitled ‘Method and apparatus for acquiring images using a CCD detector array and no transverse scanner’. This provides a 2D en face image, with depth resolution provided by scanning the reference mirror as usual in time domain OCT. Swept source spectral domain OCT can be parallelised in similar fashion, as described on Bonin et al ‘In vivo Fourier-domain full-field OCT of the human retina with 1.5 million A-lines/s’, Optics Letters 35(20), 3432-3434 (2010). Parallelisation of spectrometer-based spectral domain OCT is somewhat more complicated, because one axis of a 2D photodetector array is occupied by the wavelength dispersion. In a configuration described in Nakamura et al ‘High-speed three-dimensional human retinal imaging by line-field spectral domain optical coherence tomography’, Optics Express 15(12), 7103-7116 (2007), cylindrical lenses are used to produce a line illumination on a sample retina and a reference mirror, and the combined return sample and reference beams dispersed with a grating and detected with a 2D CMOS camera. A Fourier transform along the spectral axis provides an A scan for each position along the illuminated line. For full 3D imaging the illuminated line is mechanically scanned in the orthogonal direction and the CMOS camera read out repeatedly.
Given the relative simplicity of its basic setup and its significant sensitivity advantage over time domain OCT, parallelised swept source OCT is extremely promising for high speed 3D imaging, which is of particular interest for in vivo imaging where motion artefacts are of concern. However camera-based swept source systems, as described in the above-mentioned Bonin et al paper for example, are in fact unsuitable for in vivo applications. Because each frame corresponds to a single wavelength, the acquisition time for each A scan is equal to the frame period times the number of k-points (wavelength samples) acquired. Even for very high speed cameras with frame rates of 100 s of kHz, this can lead to A scan acquisition times of many ms which can lead to motion artefacts especially with in vivo samples. Relative movement of the sample during the A scan acquisition time will be equivalent to an error in wavelength and will degrade the image. For example in 10 ms a shift of only 0.2 μm in sample position will create a π phase error in reading between the extremes of wavelength.
OCT image acquisition speed can be improved by parallelising the detection channels as disclosed in O. P. Kocaoglu et al ‘Adaptive optics optical coherence tomography at 1 MHz’, Biomedical Optics Express 5(12), 4186-4200 (2014), but this is a complicated scheme involving multiple spectrometers accessed sequentially via an optical switch assembly.
There is a need therefore for a parallelised or multichannel optical receiver that improves the image acquisition speed of OCT, in particular swept source OCT, with less complexity than in the prior art.